One problem in industrial water treatment is the prevention of calcium scales caused by the precipitation of calcium salts, such as calcium carbonate, calcium sulfate and calcium phosphate. These salts are inversely soluble, meaning that their solubility decreases as the temperature increases. For industrial applications, where higher temperatures and higher concentrations of salts are present, this usually translates to precipitation occurring at the heat transfer surfaces. The precipitating salts can then deposit onto the surface, resulting in a layer of calcium scale. The calcium scale can lead to the loss of heat transfer in the system and cause overheating of production processes. In addition, scaling can promote localized corrosion. Calcium phosphate, unlike calcium carbonate, is not generally a naturally occurring problem. However, ortho-phosphates are commonly added to industrial systems (and sometimes to municipal water systems) as a corrosion inhibitor for ferrous metals, usually at levels between 2.0–20.0 mg/L. Therefore, the precipitation of calcium phosphate not only can result in the scaling problems previously discussed, but can also result in severe corrosion problems, as the ortho-phosphate is removed from solution. As a result, industrial cooling systems require periodic maintenance in which the system must be shut down, cleaned and the water replaced. Lengthening the time between shutdowns saves costs, and is desirable.
One way to lengthen the time between maintenance is through the use of polymers that can function by either inhibiting the formation of the calcium salts, or by modifying the crystal growth so that the salt disperses when it precipitates rather than depositing on heat transfer surfaces. Dispersion of the precipitated salt crystals is believed to be the result of the adsorption of the inhibitor onto precipitated crystals. The adsorption of the inhibitor can also be used to stabilize the system by facilitating the dispersion and subsequent removal of other suspended particulates, such as mud, silt and clay, and metals such as iron and zinc and their insoluble salts, from aqueous systems. The inhibitor may also interfere with and distort the crystal structure of the scale making the scale less adherent to surfaces or other forming crystals or existing particulates.
Currently different polymers are used to control different types of scale. Maleate-based polymers, along with phosponates are used to control calcium carbonate scale. Sulfonate-based polymers, such as those disclosed in U.S. Pat. No. 4,711,725, are used to control calcium phosphate, iron, and other particulates. U.S. Pat. No. 5,277,823 describes a method for controlling silica scale formation using a terpolymer of (meth) acrylic acid or maleic acid, a (meth)acrylamido methylpropane sulfonic acid or styrene sulfonic acid, (meth)acrylamide, and another vinyl monomer. The disclosure fails to recognize the importance of both a mono- and a di-carboxylic acid, and does not disclose a polymer for inhibiting both calcium carbonate and calcium phosphate scale.
Multi-functional scale inhibitors have focused on terpolymers polymerized from a strong acid monomer, a weak acid monomer, and a non-ionic monomer. Examples of these polymers include those polymerized from a) (meth)acrylic acid, b) sulfonic acid or acrylamidomethylpropanesulfonic acid, and c) acrylamide, sulfonated styrene, phosphinocarboxylic acid, or (meth)acrylates. These scale inhibitors tend to have only moderate calcium carbonate inhibition.
The cooling water treatment industry is moving toward a greater degree of water reuse, and higher operating cycles to conserve energy and water. To achieve this goal, increased demands are placed on cooling water treatment programs. These stressed systems contribute to a high tendency to form a variety of scales and deposits in the system. Stressed conditions and their potential negative effects include:
Stressful conditionPotential Scales/DepositsHigh pHCaCO3, Ca3(PO4)2, Fe(OH)3High hardnessCaCO3, Ca3(PO4)2High alkalinityCaCO3, ZnCO3High phosphateCa3(PO4)2High sulfateCaCO3, FeSO4High chlorideFeCl3High ironFe(OH)3High silicaSiO2, MgSiO3High suspended solidsParticulateHigh organicsMicrobiological foulingHigh temperatureCaCO3, Ca3(PO4)2, Fe2O3High conductivityFe2O3Oxidizing biocidesFe2O3Low flowParticulate, Fe2O3
It is an objective of the current invention to provide a single, multifunctional polymer capable of inhibiting both calcium carbonate and calcium phosphate scaling. It is also an objective of the present invention that the single multi-functional scale inhibiting polymer be able to perform at a treatment level equal to or below the combined treatment level of the two or more polymers presently required for the same performance, which would generally not exceed 15.0–20.0 ppm active product. It is a further objective to provide a single, multi-functional polymer that is effective under stressed conditions.
Surprisingly it has been found that a multifunctional polymer containing a dicarboxylic acid, a mono-carboxylic acid, a non-ionic monomer, and a sulfonated or sulfated monomer, in the proper ratios, provides excellent scale and deposit control. This is especially true for systems under stressed conditions. The polymer inhibits calcium carbonate and calcium phosphate scale formation and stabilizes iron and zinc, even at high dissolved and suspended solids levels.